Paragon 2015 Smash Wii U Top 8:
1. EMP|Mew2King (
Diddy Kong)
2. VS|NickRiddle (Zero Suit Samus)
3. STVG|StaticManny (
Sonic)
4. Xaltis (
Rosalina & Luma, Peach)
5. Player-1 (
Diddy Kong, Villager)
5. Myran (Olimar)
7. Leo Heart
(Diddy Kong)
7. ChoZox (Samus)
9. RyoGuikido
9. CoG|Wizzrobe (
Sheik)
9. Leffen (
Diddy Kong)
9. SSHQ|StreetShark (
Rosalina & Luma)
13. Poltergust
13. ScaryLB59
13. STVG|Phuzix
13. CFS|TrueBlue (
Sonic)
Out of all of the characters you mentioned, Diddy is most prevalent. The
only Sheik in the top 8 is from Wizzrobe, currently one of the best Sheiks in competitive play. In the top 32, there were a total of
three Sheiks that I saw. There are two Sonics, though their placings are pretty different and again, there weren't as many Sonics as a lot of people don't seem to find him enjoyable to play (or watch, because he's campy and boring). Rosalina's placing is due to Xaltis being highly skilled with her, and he still had to switch to Peach for some match-ups because Rosalina simply couldn't compete (I believe Olimar and Sonic). Sonic's placings can largely be due to how unreadable he is.
Then, there's Diddy. The major difference here is that out of the Sheiks, Rosalinas, and even Sonics I saw (though Sonic to a lesser extent), there were differences in playstyle. Wizzrobe relies heavily on fair combos and surprise attacks with bouncing fish, while MasterRaven uses more throw mix-ups and uses vanish often due to its ability to kill. Sonic play had minor differences, and I didn't have the chance to see the other Rosalina's play, though considering he actually placed quite a bit below Xaltis, he was either doing something wrong or should have had a secondary, as Xaltis did.
Looking at the Diddy players, the playstyles are entirely the same, because there's currently a "best way" to play Diddy. Grab > D-Throw > As many Uairs as you can get out. Side-B on attacks with lag (Zero Suit's down-smash was punished heavily by this). Fair and Bair in case of Uair whiff because they cover so much ground. Etc.
If competing players understand match-ups, especially as Mew2King does, Diddy is
clearly the best. Most of his attacks out-range or out-prioritize the characters you mentioned, he's faster than most of them, has better killing power and knockback than all of them, has less lag than most of them for most moves, has better throws than all of them for the most part, can play both offensively and defensively with major success, and takes longer to kill than all of them, excluding maybe Rosalina.
After watching the top 32 of Paragon, it is unquestionable that Diddy essentially belongs in his own tier above the other high tier characters. A lot of Diddy players had a lot of success doing the same thing as every other Diddy player.
For instance, in a Captain Falcon vs Diddy match, CF landed multiple d-throws into nairs, as well as some jabs, and got Diddy to 36%. Diddy then landed a d-throw, followed by 3 Uairs, and got CF to 42%. Smash 4, as Mew2King said, is almost entirely fundamentals. Meta is required for lower tier characters like ChoZox's Samus (hype matches btw), but is hardly needed for Diddy. Things are simply easy to do with him.
Also, as @
FlipFlopMist
said, Diddy is a pretty predictable character. It's fairly easy to know what he's going to throw at you, because in most situations he'll have one or two options that are most viable. The difference between him and other members of the cast is that those one or two viable options are almost certain to be better than your most viable option at that moment.
If anything, Sonic provides a tough MU for Diddy only because he's nearly impossible to read, and since Diddy's path to success relies heavily on grabs, the fact that Sonic's moves will connect with shield and then continue past make him hard to grab as well. Still, the MU is in Diddy's favor most games.
Considering M2K mains Diddy and will be at Apex, I doubt Sheiks will be making it very far. Also, looking at Paragon results, there's only 1 Sheik in the top 8, and 4 Diddys. In the top 32, this extends to 3 Sheiks and even more Diddys. I'd hardly call Diddy and Sheik even. Diddy has just as much combo potential as Sheik, plus more kill power, plus being harder to kill, plus a gimp item in the banana, plus his side-b being a safer approach option than bouncing fish. Sheik certainly has a better recovery than Diddy, but she'll hardly ever get to use it because most of Diddy's kills are off of the top of the screen.